The invention relates to a method of roll grinding and to abrasives tools used in roll grinding.
Roll grinding is a cylindrical grinding process wherein a bonded abrasive wheel grinds and smoothes the surface of a mill roll. A mill roll is a large (e.g., 7 feet in length, 2 feet in diameter) metal roller, typically made of forged steel, designed for use in the surface finishing of metal sheets. In grinding the surface of the mill roll, the grinding wheel must impart a uniform, smooth surface finish to the roll. Any imperfection, such as grinding patterns, feed lines, random marks, indentations, and the like, created on the roll surface during the grinding process will be transferred onto the metal sheets being processed by the roll.
With unstable grinding systems, the grinding conditions cause the vibration amplitude between the grinding wheel and the workpiece to increase over time. This results in a series of undulations that develop and build along the surfaces of both the grinding wheel and workpiece. This process is referred to as regenerative or self-excited chatter and has been associated with certain imperfections in the surface of mill rolls following grinding (“chatter marks”). Roll grinding operators want “chatter-resistant” grinding wheels, having the capacity to stay in a round shape and maintain a resilient character as grinding progresses and the wheel is worn. Grinding vibration models have been developed (Inasaki I., Grinding Chatter—Origin and Suppression, CIRP Proceedings, 2001) to explain the relationship between wheel properties (such as decreased contact stiffness, increased damping) and the suppression of self-excited chatter.
The roll grinding industry typically employs shellac bonded grinding wheels to minimize roll damage during grinding. In roll grinding wheels, shellac resin bonds are preferred for their relatively low elastic modulus (e.g., 1.3 GPa versus 5–7 GPa for phenolic resin bonds). Among the organic bonds used commercially in the manufacture of grinding wheels, phenolic bonds are preferred for strength, cost, availability, and manufacturing considerations. In contrast, shellac resins are natural materials collected from insects, are relatively costly, inconsistent in composition and quality, and more difficult to use in wheel manufacturing. Among the various types of organic bonded grinding wheels, shellac bonded wheels are characterized by relatively low mechanical strength, expressed as a relatively low “burst speed” (the rotational speed at which centrifugal force causes the wheel to fly apart), and as a shorter wheel life. In roll grinding operations, shellac wheels are limited to lower wheel rotation speeds (e.g., 4000 to 8000 sfpm) and shorter wheel life. The operation of the shellac wheel is troublesome, requiring frequent adjustments in wheel speed, in-feed rate and other parameters to avoid chatter as the wheel diameter is reduced by wheel wear and vibration amplitude changes.
As an alternative to shellac wheels, it has been suggested in U.S. Pat. No. A-5,104,424 to use a combination of silicon carbide and sintered sol gel alumina grains in a high elastic modulus bond wheel to control the shape of the roll surface during grinding. This tool design has not been commercially useful.
Thus, there remains a need in the industry for better abrasive grinding tools and grinding procedures suitable for manufacture and reconditioning of mill rolls having a high quality surface finish delivered at an effective operational cost.
It has been discovered that unique grinding wheels made with common abrasive tool components, such as phenolic resin bond, and conventional alumina grain, preferably that has been agglomerated with selected binding materials, can be employed to yield more efficient roll grinding processes than the best known commercial roll grinding processes.